1. Field of the Invention
The present invention relates to an image formation apparatus, and more particularly to an image formation apparatus such as a copy machine, a printer, a facsimile apparatus or the like which exposes a photosensitive body by driving a laser beam source on the basis of a detection signal obtained by detecting a laser beam from the laser beam source to form a latent image on a photosensitive face of the photosensitive body.
2. Related Background Art
In an image formation apparatus or the like which performs digital optical communication or an electrophotographic process, a laser diode is used as a light emission element to convert an electrical pulse signal into an optical pulse. For this laser diode, it is required to be able to obtain the desired light emission quantity even if the operation temperature of the element changes. However, since the light emission characteristic of the laser diode highly depends on the operation temperature, it is necessary to control a laser diode drive current by a light emission element drive circuit so as to obtain the desired light quantity even if the operation temperature changes.
As a first conventional example, FIG. 30 shows the structure of the laser diode drive circuit which performs the pulse current control on a laser of cathode drive type.
In FIG. 30, numeral 3001 denotes a comparator, numerals 3002 and 3006 denote reference voltage sources, numeral 3003 denotes a sample-and-hold (S/H) circuit, numeral 3004 denotes a hold capacitor (CH), numeral 3005 denotes a current amplification circuit, numeral 3008 denotes a reference current source (IO), numeral 3007 denotes a switching circuit (SW), numeral 3011 denotes a laser diode (LD), numeral 3012 denotes a photodiode (PD), and numeral 3010 denotes a monitor resistor (RM).
In the conventional example shown in FIG. 30, for the sampling state of the S/H circuit 3003 (referred as APC (automatic power control) operation hereinafter), the switching circuit 3007 is ON, and input data (DATA) is set such that the laser diode 3011 is in its entire-face light emission state. In the APC operation, the light quantity from the laser diode 3011 is monitored at the photodiode 3012 such that the light emission quantity of the diode 3011 becomes the desired quantity. Then, if a monitor current IM produced at the photodiode 3012 flows in the monitor resistor 3010, a monitor voltage VM is produced at the end of the monitor resistor 3010. Further, the laser diode drive current is controlled by the current amplification circuit 3005 on the basis of the reference current source 3008 such that the monitor voltage VM becomes constant (i.e., light emission quantity becomes constant).
Further, during the hold of the S/H circuit 3003, the laser diode drive current is set to be ON/OFF by the switching circuit 3007 according to the input data, whereby the pulse modulation signal is given to the laser diode 3011.
However, in the structure shown in FIG. 30, if the operation frequency in the optical pulse modulation becomes high, the problem of light emission delay which is peculiar to the laser diode occurs, whereby the transition characteristic of the modulated optical pulse deteriorates.
FIG. 31 shows a second conventional example relating to one method to solve the above problem of the first conventional example. In the second conventional example, a DC bias current is added to the laser diode drive current to improve the transition characteristic of the optical pulse which has been deteriorated by the light emission delay of the laser diode. Since the basic structure of the second conventional example is the same as that of the first conventional example shown in FIG. 30, the detailed explanation thereof is omitted. In FIG. 31, numeral 3009 denotes a current source (IB) which produces the bias current, and numeral 3015 denotes a reference pulse current source (IPO).
Also, in the second conventional example of FIG. 31, during the APC operation, the switching circuit 3007 is ON, and the input data is set such that the laser diode 3011 is in its entire-face light emission state. A pulse current IP is controlled by the current amplification circuit 3005 according to the reference pulse current IPO, on the basis of the monitor voltage VM obtained by the structure consisting of the photodiode 3012 and the monitor resistor 3010, so that the light emission quantity of the laser diode 3011 becomes constant in the entire-face light emission state. Then, a laser diode drive current ILD is determined by superimposing the pulse current IP on the bias current IB.
Further, during the hold of the S/H circuit 3003, the pulse current IP is set to be ON/OFF by the switching circuit 3007 according to the input data, whereby the pulse modulation signal is given to the laser diode drive current ILD.
In the second conventional example of FIG. 31, if the bias current IB is not added nearly up to the threshold current from which the laser diode 3011 starts light emission, it is impossible to effectively lower the light emission delay of the diode 3011.
However, in the second conventional example, as described above, the oscillation threshold current of the diode 3011 changes due to the operation temperature. Also, such the current changes according to respective elements. For these reasons, since the optical pulse does not completely come to be OFF, there is every possibility that a sufficient quenching ratio can not be obtained. Therefore, it is difficult in the practical use to set the bias current to be the fixed value nearby the threshold current.
FIG. 32 shows a third conventional example. Like the second conventional example, the third conventional example relates to the method in which the bias current is added to the drive current. However, the current to be controlled is the bias current while the pulse current is given as the fixed current. In FIG. 32, it should be noted that the detailed explanations of the parts added with the same reference numerals as those in FIG. 30 are omitted. In the drawing, numeral 3013 denotes a reference bias current source (IB0) which determines a bias current IB, and numeral 3014 denotes a pulse current source (IP) which produces the pulse current IP.
Also, in the third conventional example of FIG. 32, during the APC operation, the switching circuit 3007 is ON, and the input data is set such that the laser diode 3011 is in its entire-face light emission state. In order that the light emission quantity of the laser diode 3011 reaches the desired value in the light emission state, the bias current IB is controlled by the current amplification circuit 3005 according to the reference bias current IB0, on the basis of the error voltage (i.e., difference voltage) between the monitor voltage VM obtained by the structure consisting of the photodiode 3012 and the resistor 3010 and a reference voltage Vref1 corresponding to the desired light quantity, thereby controlling the laser diode drive current ILD. Further, during the hold of the S/H circuit 3003, the pulse current IP is set to be ON/OFF by the switching circuit 3007 according to the input data, whereby the pulse data is given to the laser diode drive current ILD to perform the optical pulse modulation.
However, in the third conventional example of FIG. 32, e.g., when the laser diode drive current may be small in such the case as the laser diode 3011 operates at low temperature, there is some possibility that the bias current becomes unnecessary and thus the operation becomes uncontrollable.
Hereinafter, the case where the operation becomes uncontrollable will be explained in detail.
FIG. 33 shows the relation between the laser drive diode current ILD and a light output P based on the change in the operation temperature of the general laser diode.
If the operation temperature rises, the threshold current increases, whereby the laser diode drive current ILD increases. In this case, the above-described problem does not occur. On the other hand, if the operation temperature lowers, the threshold current decreases. Thus, since the current ILD may be small, the bias current IB is decreased to set the light output P to have the desired value. However, in such the state as the bias current IB is unnecessary and also the desired light output can be obtained at the value lower than the setting value of the pulse current IP, it is impossible to control the light quantity to be constant. This is because the pulse current IP being the fixed value can not be set to be equal to or lower than the setting value.
Further, in the temperature characteristic of the laser diode, there is the specific phenomenon that the slope efficiency (also called as differential efficiency) is lowered in the laser oscillation area. Thus, in the optical pulse modulation, in the case where the laser diode drive current is increased due to increase of the operation temperature or the like and thus the slope efficiency is lowered, it is impossible to sufficiently secure the quenching ratio of the laser diode if the pulse current is not increased.
FIG. 34 shows the relation between the change of the temperature and the change of the laser drive current ILD and also shows the ratio of the pulse current to the bias current.
If an operation temperature Ta is lowered, the laser drive current ILD decreases. Even if the operation temperature Ta is further lowered, it is possible to set the light emission quantity of the laser diode to be constant until the bias current IB reaches "IB=0". However, if the bias current becomes unnecessary, the control becomes impossible. That is, in an area A of FIG. 34, the current to be used to decrease the pulse current IP is necessary, whereby it is impossible to perform the control for obtaining the desired light quantity.
As described above, in the conventional technique, in order to secure the high-speed light emission operation of the laser diode, the DC current close to the oscillation threshold current has been previously supplied as the bias current. Then, the pulse current according to the input data is superimposed on the bias current, and the obtained current is supplied to the laser diode. In this operation, as the methods to drive the light emission operation of the laser diode, there are the pulse current control and the bias current control, and each control has its merits and demerits.
In the bias current control, the high-speed optical pulse modulation can be secured. However, there is every possibility that sufficient quenching ratio of the laser beam can not be obtained due to the changes of the oscillation threshold current and the slope efficiency that occur by the operation temperature change of the laser diode or the like. Further, in the state that the bias current is equal to or lower than "0", it is impossible to perform the control for obtaining the desired light quantity. On the other hand, in the pulse current control, if the bias current is set not to exceed the threshold current in any operation temperature, the quenching ratio of the laser beam can be sufficiently secured. However, in such a case, when the optical pulse of high frequency is modulated, the transition characteristic of the optical pulse is deteriorated.
FIG. 35 shows the laser diode drive circuit used in a fourth conventional example.
In the drawing, numeral 3501 denotes a laser diode, and numeral 3502 denotes a photodiode which monitors the light emission quantity of the laser diode 3501. A controllable bias constant current source 3526 is connected to the laser diode 3501, and a controllable light emission constant current source 3527 is also connected to the diode 3501 through a switching circuit 3528 which generates the pulse modulation signal according to the input data. One end of a resistor r is connected to the output of the photodiode 3502, and the other end thereof is grounded. Further, a low-level sample-and-hold (S/H) circuit 3529 which samples and holds the quantity of the light emitted by the bias current and a high-level sample-and-hold (S/H) circuit 3530 which samples and holds the quantity of the light emitted by the light emission current are connected to the output of the photodiode 3502. Furthermore, the switching circuit 3528, the low-level S/H circuit 3529 and the high-level S/H circuit 3530 are controlled by a control circuit 3531.
Initially, in order to determine the bias current IB, the control circuit 3531 sends the control signal to set the switching circuit 3528 to be nonconductive. Then, the light emission quantity monitored by the photodiode 3502 is sampled and held by the low-level S/H circuit 3529, the obtained quantity is compared with the low-level reference light quantity value, thereby controlling the bias constant current 3526 to obtain the desired low-level light quantity.
Subsequently, the light emission current is determined. At this time, the control circuit 3531 sends the control signal to set the switching circuit 3528 to be conductive, and thus the light emission current is controlled in the state that the bias current flows. The light emission quantity monitored by the photodiode 3502 is sampled and held by the high-level S/H circuit 3530, the obtained quantity is compared with the high-level reference light quantity value, thereby controlling the light emission control current source 3527 to obtain the light quantity capable of giving the desired high level.
When the optical pulse modulation is intended to be performed on the laser diode, the switching circuit 3528 may be turned on/off according to the modulation data. Thus, the bias current is applied in the low level pulse modulation in the state that the circuit 3528 is OFF.
FIG. 36 shows the laser diode drive circuit used in a fifth conventional example.
Numeral 3501 denotes the laser diode, and numeral 3502 denotes the photodiode which monitors the light emission quantity from the laser diode 3501. The output of the photodiode 3502 is connected to a control circuit 3531. The controllable bias constant current source 3526 is connected to the laser diode 3501, and the controllable light emission constant current source 3527 is also connected to the diode 3501 through the switching circuit 3528 which generates the pulse modulation signal according to the input data.
Numeral 3531 denotes the control circuit which is composed of, e.g., the CPU and the like. The control circuit 3531 sends the control signal to a latch circuit 3532 for determining the bias current and also to a latch circuit 3533 for determining the light emission current. Numerals 3534 and 3535 respectively denote digital-to-analog (D/A) conversion circuits. The D/A conversion circuit 3534 converts the digital data held by the latch circuit 3532 into the analog data to supply the data for the output control of the bias constant current source 3526. On the other hand, the D/A conversion circuit 3535 converts the digital data held by the latch circuit 3533 into the analog data to supply the data for the output control of the light emission constant current source 3527.
FIG. 37 is the characteristic diagram showing the relation of the laser drive current and the laser light emission quantity.
If it is externally instructed to set the light emission quantity of the laser diode 3501 to be the desired light quantity, the control circuit 3531 initially sets the switching circuit 3528 to be nonconductive to determine the current value of the bias constant current source. Then, the circuit 3531 sends the control data signal to the latch circuit 3532 and the D/A conversion circuit 3534 to stepwise increase the bias current. After then, the bias current is stepwise increased, and the output current of the bias constant current source at the time when the light emission quantity of the laser diode 3501 is abruptly increased is considered as an oscillation threshold current Ith. Thus, the bias current source is controlled by correcting the current Ith.
Alternatively, in the circuit structure same as that shown in FIG. 36, the calculation means for obtaining the oscillation threshold current is provided in the control circuit 3531. Thus, in order to determine the current value of the bias constant current source, the circuit 3531 sets the switching circuit 3528 to be nonconductive and sends the control data to the latch circuit 3532 and the D/A conversion circuit 3534 to stepwise increase the bias current. If the light emission quantity of the laser diode 3501 reaches a predetermined first light emission quantity P1, the circuit 3531 stores bias current control data I1 obtained at that time into its internal memory. Also, the circuit 3531 increases the light emission quantity to increase the bias current up to a second light emission quantity P2, and stores bias current control data I2 obtained at this time into its internal memory.
Subsequently, the control circuit 3531 resets the data of the latch circuit 3532 and once stops the bias current control. Then, the circuit 3531 calculates the oscillation threshold current Ith of the laser diode 3501 from the bias current control data I1 for the first light emission quantity P1 and the bias current control data I2 for the second light emission quantity P2 on the basis of a following equation (I). ##EQU1##
However, even if the oscillation threshold current Ith obtained based on the equation (I) is applied to the laser diode 3501 as the bias current, the diode 3501 is not sufficiently quenched. That is, as shown in FIG. 37, it has been known that the diode 3501 emits the light of light quantity Pth. For this reason, e.g., in the electrophotographic process, there is every possibility that such the light quantity produces undesirable background on the image.
Therefore, the control circuit 3531 has the means to correct the oscillation threshold current Ith on the basis of a following equation (II) or (III), thereby determining the bias current IB. EQU IB=Ith.times..alpha.(0.ltoreq..alpha..ltoreq.1) (II) EQU IB=Ith-Ix(0.ltoreq.Ix.ltoreq.Ith) (III)
If the bias current IB is determined by the above means, the control circuit 3531 sends the digital data to the latch circuit 3532 such that the output current value of the bias constant current source 3526 has the value IB. Then, the D/A conversion circuit 3534 converts the digital data held by the latch circuit 3532 into the analog data, to control the bias constant current source 3526 and apply the bias current IB to the laser diode 3501.
Subsequently, the control circuit 3531 sets the switching circuit 3528 to be conductive, and sends the control data signal to the latch circuit 3533 to increase the light emission current up to the desired light emission quantity. That is, the current of the light emission constant current source 3527 is supplied to the laser diode 3501 in the state that the bias current IB is applied by the bias constant current source 3526. Then, if the light emission quantity of the diode 3501 reaches the desired light quantity, the circuit 3531 stops to increase the light emission current. The light emission current control data value at this time is held by the latch circuit 3533. When the optical pulse modulation is intended to be performed on the laser diode 3501, the switching circuit 3528 may be turned on and off according to the modulation data. Thus, the bias current is applied in the low-level pulse modulation in the state that the circuit 3528 is OFF.
However, the following problems have occurred in the conventional laser diode drive circuit.
In such the method as in FIG. 35 that the light emission quantity only by the bias current is monitored and the bias current is controlled based on the monitored quantity, it is impossible to completely make the low-level light quantity "0". Therefore, such the method may be used to perform the high- and low-level pulse modulation. However, if such the method is used in the electrophotographic process, it is desirable to restrict the low-level light quantity as little as possible. That is, if the light is produced from the laser diode in the low-level pulse modulation, there is every possibility that such the light quantity produces undesirable background on the image. This is the serious problem.
Further, when the very small low-level light quantity is detected, there is every possibility that it is resultingly impossible to stably control the bias current based on the output signal from the photodiode 3502.
On the other hand, in such a laser drive circuit as in FIG. 36, the bias current is stepwise increased by using the D/A conversion circuit to determine the bias current. Thus, in order to perform the accurate control, it is necessary to improve the resolution of the D/A conversion circuit. Especially, the serious problem due to high resolution occurs when the laser drive current is increased or the slope efficiency in the laser light emission area is increased. For example, if such the drive circuit is used in the electrophotographic process, it becomes difficult to output the halftone image.
Further, in such the control operation, since the current value is stepwise increased from the natural light emission area to the laser light emission area, a long operation time is necessary until the bias current and the light emission current are determined. Thus, the high-speed clock signal is necessary to terminate the control as quickly as possible.